The present invention relates to organic matrixes containing dispersed high refractive index nanosize metal oxide particles, and to processes of making nanosize titanium-based oxide particles which, in part, may be used to increase the refractive indexes of transparent organic matrixes.
Some optical applications require transparent materials having high refractive indexes. Polymers with refractive indexes less than that required for certain optical applications may be utilized if modified by partial substitution of atoms, such as bromide or sulfur, for hydrogen or oxygen in the polymers. Such substituted polymers typically have increased refractive indexes but are often undesirably colored and lack thermal and photochemical stability. Therefore, these substituted polymers may not be the best choice for certain optical applications.
An alternative method of increasing the refractive indexes of polymers is combining metal oxide particles with polymers to form ceramers. Ceramers may be defined as hardened or cured compositions having ceramic particles embedded or grafted into the polymer matrixes and typically having optical and physical characteristics intermediate between those of the metal oxide and the organic component. Transparency of ceramers is dependent upon, in part, the sizes and refractive indexes of the metal oxide particles contained therein. If the metal oxide particles used in ceramer preparations have large mean diameters, the transparency of the ceramers may be diminished. Light directed at the ceramer containing large metal oxide particles would be reflected back to the light source or deflected to the side, reducing the apparent transparency of the ceramer. Moreover, the transparency of the ceramer may be diminished upon the addition of very small metal oxide particles, if the particles agglomerate within the polymer. These particle agglomerates act as larger particles that may scatter or reflect light. In addition, during the preparation of ceramers, metal oxide particles may precipitate creating what appears to be separate layers(s) that have less than maximal transparency. For a transparent ceramer, the small metal oxide particles must be highly dispersed (non-agglomerated) in the polymer to avoid light scattering.
One method of ceramer preparation is to treat metal oxide particles with specific organic acids, or other surface active agents, before combining the metal oxide particles with polymer precursors. It is through that surface-active agents coat the surface of the metal oxide particles and create an organophilic interface with the polymer compositions. An adsorbed organic layer on the outside of the metal oxide particles sterically inhibits agglomeration resulting in greater metal oxide particle dispersion stability. Another method of increasing the dispersibility of metal oxides in polymer compositions is to electrostatically stabilize the metal oxide particles by forming aqueous colloids. The electrostatically charged layer surrounding the metal oxide particles provides repulsive forces between metal oxide particles inhibiting agglomeration. These methods of dispersing metal oxides are not perfect. Consequently, transparent polymer/metal oxide compositions having high refractive indexes are desirable but difficult to obtain because the metal oxide particles, when combined with polymer precursors, may agglomerate and decrease the transparency of the resulting ceramers.
The refractive indexes of ceramers or ceramer compositions are, in part, dependent upon the refractive indexes of the metal oxide particles added to the organic matrix. The theoretical refractive index of a single ceramer can only be as high as the volume weighted average of the refractive indexes of the metal oxide particles and the polymer matrix. Consequently, it is desirable to make ceramers using metal oxide particles having high refractive indexes. Metal oxide particles in crystalline form typically have higher refractive indexes than metal oxide particles that are amorphous (that is, non-crystalline). Therefore, using crystalline metal oxides to make ceramers is desirable, though highly crystalline metal oxide particles that are dispersible are difficult to make because of their tendency to agglomerate in polymer compositions. Generally for a given particle size, the tendency to flocculate increases with increasing refractive index of the particles. For most high refractive index oxides, the form that is precipitated from solution is usually an amorphous or lower refractive index form. Treatments to induce crystallization (for example, heating at an elevated temperature) often tend to induce flocculation or agglomeration of the metal oxide particles. Heating also tends to induce sintering of aggregated of primary particles, especially very small particles that have very high surface areas and thus higher total surface energies. Because of its high dielectric constant, water is an excellent medium for crystallizing and stabilizing metal oxide particles. However, metal oxide particles that are formed in water are difficult to transfer into organic liquids without particle agglomeration and concomitant transfer of water.
The present invention embodies metal oxide particles that may be combined with transparent polymer compositions without substantially diminishing the transparency of the ceramer compositions or ceramers produced. The metal oxide particles have dispersing aid attached thereto and have a degree of crystallinity of greater than about 55 percent. The metal oxide particles also have a particle size, or crystallite diameter, greater than about 4 nanometers and less than about 20 nanometers, and an absorptivity of less than 3.00 as an organic colloid. The metal oxide particles are preferably titanium-based particles.
The present invention embodies transparent ceramers and ceramer compositions comprising metal oxide particles of the present invention dispersed in an organic matrix. The transparent ceramers of the present invention typically have a refractive index greater than about 1.6. The organic matrix of the ceramer is preferably prepared from polymer precursors selected from the group consisting of molecules possessing ethylenically unsaturated polymerizable functionality and combinations thereof.
The present invention embodies colloids including the metal oxide particles of the present invention having dispersing aid attached thereto dispersed in an organic liquid. These colloids are substantially free of particle agglomeration and of water.
The present invention embodies coated articles having a substrate with a surface attached to a coating. The substrate, the coating, or both may comprise a ceramer of the present invention. The substrate and coating may comprise the same ceramers or different ceramers.
The present invention embodies a process of making dispersible crystalline metal oxide nanoparticles. The process includes (a) providing metal alkoxides, and (b) reacting the metal alkoxides with a substiochiometric amount of a complexing agent including a carboxylic acid having a carbon chain of about 3 carbon atoms to about 18 carbon atoms. The process also includes the steps of partially hydrolyzing the product of step (b) in step (c) by the addition of substoichiometric amounts of water and (d) thermally treating the partially hydrolyzed mixture by heating under pressure at a temperature in the range of about 150xc2x0 C. to about 265xc2x0 C. for an amount of time sufficient to form crystalline particles having a degree of crystallinity greater than about 50 percent and less than about 86 percent. Typically the partially hydrolyzed mixture is thermally treated under 18 atmospheres to 40 atmospheres of pressure.
As used herein, with respect to the present invention, the following shall apply:
xe2x80x9cAbsorptivityxe2x80x9d refers to the inhibition of light transmission by scattering or light absorption. Absorptivity as used here is measured by an absorptivity test procedure described below.
xe2x80x9cAutogenousxe2x80x9d refers to self-generated. For example, autogenous pressure results when a vessel is sealed at atmospheric pressure and then heated. The components within the vessel volatilize and form gas within the vessel increasing the pressure within the vessel.
xe2x80x9cCeramerxe2x80x9d refers to a ceramer composition in which a curable composition is cured to form a solid, substantially non-flowing material.
xe2x80x9cCeramer compositionxe2x80x9d refers to a coatable dispersion comprising substantially non-aggregated, colloidal inorganic oxide particles dispersed in a curable organic binder composition, wherein curing of the binder is understood to mean in a board sense the process of solidification (hardening) of the binder brought about by a suitable method such as cooling of a molten thermoplastic material, drying of a solvent-containing composition, chemical crosslinking of a thermosetting composition, radiation curing of a radiation curable composition, or the like.
xe2x80x9cColloidxe2x80x9d refers to a stabilized dispersion of metal oxide particles in a liquid.
xe2x80x9cCoupling agentxe2x80x9d refers to an organic composition having a first functional group capable of covalent or ionic bonding to a metal oxide particle, a second functional group capable of reacting with an organic precursor and optionally a hydrophobic segment located between the first and second functional groups.
xe2x80x9cCurablexe2x80x9d means that a coatable material can be transformed into a solid, substantially non-flowing material by means of cooling (to solidity hot melts), heating (to dry and solidify materials in a solvent), chemical crosslinking, radiation crosslinking, or the like.
xe2x80x9cDiscretexe2x80x9d refers to non-agglomerated primary particles.
xe2x80x9cDispersant or Dispersing Aidxe2x80x9d refers to an organic composition having a first functional group capable of covalent or ionic bonding to a metal oxide particle and a hydrophobic segment. The term dispersant or dispersing aid includes coupling agent(s) that have a second functional group.
xe2x80x9cOrganophilicxe2x80x9d refers to dispersible in non-polar organic solvent.
xe2x80x9cSeedxe2x80x9d refers to preexisting particle(s) utilized as a growth site to form a colloidal particle by precipitation.
xe2x80x9cSubstiochiometric amountxe2x80x9d refers to 2 moles of water of less added per mole of metal atoms in metal alkoxide. In case of titanium alkoxide, (Ti(OR)4, a substiochiometric amount of water would be less than 2 moles of water per mole of titanium.
xe2x80x9cTitanium based oxide particlesxe2x80x9d refers to metal oxide particles comprising an oxygen anion lattice wherein greater than or equal to 33 mole percent of the cations in the metal oxide are titanium cations.
xe2x80x9cThermally treatingxe2x80x9d refers to the process of making the metal oxide particles of the present invention in which the process includes heating the partially hydrolyzed metal alkoxide under pressure.
xe2x80x9cUniformxe2x80x9d refers to a narrow distribution of particle sizes of the metal oxide particles as measured by light scattering or transmission electron microscopy. The metal oxide particles of the present invention are uniform in that they have a particle size typically greater than 4 nanometers and smaller than 15 nanometers.